Help troubleshooting O2 sensor problem...
 

So, I should probably be double-checking the wiring of my 4-wire O2 sensor before posting this, but the other day I left my parking lights on, drained my battery, and I got a code 17 check engine light (O2 signal not changing) on my way home from getting jumped. So, my guess is the problem has actually existed for some time, I never noticed the check engine light the first time, but when my battery drained, it cleared the codes and threw the light on again.

Anywho...what's happening is that, as the engine and O2 sensor warm up, the voltage starts rising, and goes to about ~1.04v, and pretty much sits there. I have one of Bruce's O2 clamps, but it appears to be working correctly, because when I blow into the vacuum line/pressure switch, it switches down to ~0.33v.

So, basically, what possible issues could any of you think of that could cause an O2 sensor to basically just get stuck at it's peak output like that (their normal operating range is 0-1v). Oh, and the symptom is identical with a brand-new O2 sensor, so I know the sensor is fine.

Do you have a WB to confirm the reading?

Sounds like your car isn't goint to closed loop. Do you pay attention to your mileage? What forces open loop... Hmm either an unrelated sensor issue as miscalibrated TPS (open loop at WOT) or bad CLT sensor (open loop when cold). But you didn't get those codes. How about break in the o2 sensor wire before it reaches the ecu's o2 sensor input?

I had an LM-1 wideband in the car for probably close to a year, about a year ago. At that time, I used the simulated narrowband output from the LM-1 to feed the ECU, and can confirm it was going into closed loop just fine for the entire time that sensor/controller was installed.

Ya know, I don't pay super close attention to mileage, but, now that you mention it, I definitely feel like the last several fill-ups had been arriving awfully fast. Good catch.

Yes, I've got to double-check the wiring of the O2 sensor as well as to the ECU, I guess. Thanks for thinking of the CLT and TPS sensors, as well.

Hmmm...this is strange. I basically only had time to re-trace the wires on my 4-wire O2 harness just to make sure that everything was wired correctly, and also confirmed that the O2 signal is making it to the ECU (checked at harness-to-ECU wire).

When you turn on the car, the O2 signal starts registering after a few seconds, works its way up to ~.9-1.0v, and just sits there. When I blip the throttle, the signal changes slightly, and when I rev/blip harder, it does drop down to 0.2-0.4v range for a split-second before going right back to 0.9-1.0v.

I can't figure out why the thing won't go into closed-loop mode. I'll have to check the TPS and CLT sensor outputs tomorrow.

Any other ideas are appreciated. BTW, this must've been related to the day recently when my battery drained, because surely I would've seen the check engine light come on at some point in the past few weeks/months...the CEL appears to come on repeatedly, at least once per drive, not just once and then storing an error code like I guessed.

I had a similiar experience when my NTK O2 sensor went bad. I put it off to the heater circuit shorting to the sensor. Try disconnecting both of the heater leads and see if it will operate as a one-wire. If that doesn't change things, pull it out and burn it off with a torch and blow it out with an air line.

Hmmm...Yeah, I didn't think about disconnecting the heater leads. I'll try that, too. Again, I think the sensor is fine, as it's brand-new as of this weekend.

Thanks for the ideas, Bruce :-)

If pulling off the heater leads shows nothing, disconnect the white lead from the O2 clamp switch on the chance that there might be something wrong with the switch and it is shorting both circuits to each other.

Well, I took both the O2 clamp and the heater wire circuit out of the equation, took the car for a spin, came back and measured the O2 voltage, and...whadd'ya know...it was holding steady in the ~.55-.65v range, so it evidently was able to make it into closed-loop mode.

Turned off the car, reconnected the O2 clamp, went for another spin, came back and measured voltage, and it was again doing the closed-loop thing. I'll leave the heater circuit out of the equation for a while until I'm sure the O2 sensor is indeed acting normally and no CELs pop up again.

So, I'm still not quite sure what happened to the old O2 sensor/what caused the whole scenario. I double-checked my wiring last night, but, it being dark and me being bitten by bugs, perhaps I was too quick to find an open ground wire from the O2 sensor?

There ya go. So add the items back in, one at a time. Would do the heater circuit first, test for closed loop, and if good then add the clamp back in. Perhaps the o2 signal wasn't making it through the clamp to the ecu

I am very glad for you that the problem wasn't too difficult, like in one of the engine sensors or wiring. I just went through diagnosing miscelanious electrical grief in my OTM and it is a nightmare.